1. Field of the Invention
This invention relates to a method of compensating for fluctuations in an image signal caused by the nonuniformity in how a photoelectric read-out means detects light emitted from (or reflected by, etc.) a recording medium, or the like, in an image read-out apparatus. In the image read-out apparatus, the recording medium, on which an image has been recorded, is exposed to a light beam in order to cause light carrying information about the image to be emitted from the recording medium, and the light is photoelectrically detected and converted into an image signal. This invention also relates to a method of compensating for fluctuations in an image signal caused by nonuniformity in the intensity of stimulating rays, nonuniformity in how a photoelectric read-out means detects light emitted by a stimulable phosphor sheet, or the like, in a radiation image read-out apparatus. In the radiation image read-out apparatus, the stimulable phosphor sheet, on which a radiation image has been stored, is exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during exposure to radiation; and the light is photoelectrically detected, and converted into an image signal.
2. Description of the Prior Art
Image read-out apparatuses, wherein a recording medium, on which an image has been recorded, is exposed to a light beam, and light reflected by the recording medium, light which has passed through the recording medium, or light emitted by the recording medium is detected in order to allow the image to be read out, have heretofore been used as, for example, scanners for plate making and input means for computers and facsimiles. The image read-out apparatuses are also used as radiation image read-out apparatuses in radiation image recording and reproducing systems, wherein stimulable phosphor sheets are used, as proposed by the applicant in, for example, U.S. Pat. Nos. 4,258,264 and 4,346,295 and Japanese Unexamined Patent Publication No. 56(1981)-11395.
Specifically, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor, which has been exposed to the radiation, is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. In the aforesaid radiation image read-out apparatuses, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is used. The stimulable phosphor sheet is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal.
In the image read-out apparatuses described above, a photoelectric read-out means is used to detect light which is emitted from the recording medium when it is exposed to a light beam and which carries information about the image recorded on the recording medium. By way of example, the photoelectric read-out means is constituted of a comparatively small photomultiplier and a light guide member, one edge face of which is positioned along a main scanning line on a recording medium, and the other edge face of which is positioned such that it is in close contact with the light receiving face of the photomultiplier. Alternatively, the photoelectric read-out means is constituted of a long photomultiplier which is positioned along a main scanning line on a recording medium, as disclosed in, for example, U.S. Pat. Application Ser. No. 141,259, or a line sensor which is positioned along a main scanning line.
However, with the image read-out apparatuses described above, the image signal generated by the photoelectric read-out means fluctuates, due to nonuniformity in how the photoelectric read-out means detects the light emitted from (or reflected by, etc.) the recording medium. The nonuniformity is caused by nonuniformity in how the light is guided by the light guide member along the main scanning direction, or by nonuniformity in the sensitivity of the long photomultiplier along the main scanning direction. In cases where the aforesaid nonuniformities cause the efficiency with which the light is detected to worsen (i.e. cause shading to occur), it becomes impossible to accurately detect the image which was recorded on the recording medium.
In order to eliminate the aforesaid problems, as disclosed in, for example, U.S. Pat. No. 4,734,783, the applicant has proposed apparatuses for eliminating shading, wherein the characteristics of the shading are detected in advance, and the image signal, the sensitivity of the photomultiplier or the like, is corrected in accordance with the position at which the light beam scan in the main scanning direction. Thereby, adverse effects from shading are eliminated.
Also, in order to detect the characteristics of the shading, a method is often employed wherein, for testing, the whole plane of a recording medium is exposed to radiation having a uniform intensity and is then scanned with a light beam before the image readout is carried out. Light emitted from the recording medium during the test scanning is detected by a photoelectric read-out means. Specifically, if the image read-out apparatus is free of shading, light emitted during the test from the whole plane of the recording medium which had been uniformly exposed to radiation can be detected as a constant. Therefore, the change in the value obtained from detection of the light emitted from the recording medium for test which has been uniformly exposed to radiation can be studied in order to detect the characteristics of the shading.
However, when the method described above is used to detect the shading characteristics, it is not possible to accurately achieve a uniform exposure of the recording medium to test the radiation. Specifically, the sources of radiation themselves generally cause the shading, and it is not always possible to obtain a source of radiation which can uniformly irradiate radiation to the whole plane of a recording medium for testing purposes. Moreover, even if a radiation source which uniformly produces radiation could be obtained, it is very difficult to fix the recording conditions for a uniform exposure. This is because nonuniformity in the distribution of radiation arises when the radiation source is located too close to the recording medium to test. In order to have the radiation source located sufficiently apart from the recording medium, a fairly large room for the recording is needed. Furthermore, the characteristics of the shading of the image read-out apparatuses change with the passage of time. Therefore, even if the characteristics of the shading of an image read-out apparatus were detected from operations wherein the recording medium used for testing has been uniformly exposed to radiation, it would become necessary for the user of the image read-out apparatus to detect the characteristics of the shading of the image read-out apparatus again after the user had used the image read-out apparatus for some period of time. In such cases, the user cannot detect the shading characteristics unless he has a source of radiation which uniformly irradiates radiation to a recording medium for testing purposes.